Ya I followed you with the epic, just saw yours and went with it lol oops.
^ directed to jhawk.
The apn will not switch automatically, the fast.t-mobile one will work for both, you could leave it all the time, however if your in an area that lte is weak switching to epc and even putting the phone to wcdma preferred will get you a better signal.
for some reason ( I think because the phone is not technically supposed to have lte) t mobile has decided not to have the nexus 4 on the whitelist for lte and so if your not on fast.t-moble the lte signal is not able to connect causing some strange behavior if you have it enabled but not using the right apn